The long term goal of this project is to determine the mechanism of peroxidation catalysis for members of a homologous family of mammalian peroxidases, each of which has a distinct physiological function. Myeloperoxidase, located in the azurophil granules of neutrophils, catalyses the oxidation of chloride ion to hypochlorite which is effective in killing bacteria, viruses and fungi. A closely related peroxidase from eosinophils primarily uses thiocyanate as substrate to produce hypothiocyanous acid which is used in killing larger metazoan parasites. Similar protective functions are associated with related peroxidases found in milk, saliva and the intestinal mucosa. In contrast, thyroid peroxidase participates in the biosynthesis of the hormone thyroxine by catalyzing the iodination and coupling of tyrosines. The X-ray crystal structure analysis of canine myeloperoxidase recently completed in my laboratory has provided the first 3-dimensional model for a member of this mammalian peroxidase gene family. Refinement of a higher resolution (2.3 Angstroms) structure of human myeloperoxidase, using the technique of simulated annealing, is expected to yield more detailed information on the nature of the unusual covalently bound heme, the conformation of residues that participate directly in catalysis and the coordination geometry of a bound calcium ion. X-ray structural analysis of a complex of human myeloperoxidase with the specific inhibitor salicylhydroxamic acid will provide a model for the binding of analogous aromatic peracid substrates. Inhibitors of myeloperoxidase are of particular clinical relevance since they may be useful in reducing tissue damage in certain inflammatory diseases. Myeloperoxidase aggravates proteolytic tissue damage in emphysema through its ability to inactivate human alpha-1 anti-proteinase. Attempts will be made to crystallize human recombinant myeloperoxidase expressed in Chinese hamster ovary cells. The availability of diffraction quality crystals would facilitate future studies of the mechanism of catalysis through X-ray structural analyses of genetically engineered mutants. As a first step toward structural studies of a second member of the mammalian peroxidase family of enzymes we propose to crystallize bovine lactoperoxidase from milk.